HSBC customers were locked out of internet banking for several hours on Friday after the company was targeted by online criminals in a denial of service attack.

The bank, which has 17 million personal banking and business customers in the UK, said its website had been attacked, but it had successfully defended its systems. Customers were unable to log into their accounts until late in the afternoon, on what is likely to have been a busy day for online banking, as many employees received their first pay packet of the year.

A denial of service attack overwhelms a website with traffic, taking it offline, and is sometimes used as a smokescreen for other attacks. The bank said there were no indications of customer data theft. It is now working with the government-backed Computer Emergency Response Team, Cert-UK, to pursue the criminals responsible.

News of the cyber-attack broke mid-morning a couple of hours after customers started reporting that they were unable to access their accounts. Shortly before 5pm, John Hackett, HSBC’s UK chief operating officer, said the bank was continuing to experience attempted denial of service attacks and was closely monitoring the situation with the authorities.

“HSBC’s internet and mobile services have partially recovered, and we continue to work to restore a full service,” he said.

“We apologise for the disruption and inconvenience this may have caused.”

Robert Capps of tech company NuData Security said distributed denial of service attacks [DDoS] were not direct attacks on the accounts held at financial institutions. “They are attacks on the public image and consumer goodwill towards those institutions,” he said. “They are meant to harass, intimidate and embarrass a targeted institution, but the DDoS attacks rarely result in any lasting impact on individual accounts at an institution.”

However, he said the attacks had been used as cover for other activities, such as cyber-heists, at a targeted institution.

“They are sometimes meant to draw away the attention of the information security teams of a financial institution from the real intent of the attacks, such as large value money transfers, or the bulk theft and removal of consumer account data.

“Only time will tell if the HSBC cyber-attack is simply a DDoS attack or a cover for a much more damaging intrusion into their systems.”

Andrew Tyrie MP, chairman of the Treasury committee, said he had recently written to regulators asking them to take action on banks’ IT systems.

“Bank IT systems just don’t seem to be up to the job. This leaves bank customers with a substandard service,” he said.

“Incidents like these are unacceptably frequent, and sometimes serious. Until this is sorted out, the public will remain more exposed than necessary to the risks of IT banking failures, including delays in paying bills, an inability to obtain their own money, and unauthorised access to their accounts.”